In LTE (Long Term Evolution), which is one of the standards for radio communication systems defined by 3GPP (3rd Generation Partnership Project), radio resources including a time domain and a frequency domain are assigned to each radio terminal (User Equipment: UE) by using the TDM (Time Domain Multiplexing)/FDM (Frequency Domain Multiplexing) scheme. With respect to uplink signals transmitted by multiple radio terminals to a radio base station (enhanced Node B: eNB) in particular, the radio base station controls the transmission timing of an uplink signal of each radio terminal so that it is accommodated within a predetermined receive window at the radio base station. This control of uplink-signal transmission timing is performed by using the following two (NPL 1).                Uplink-signal transmission timing adjustment value (Timing Advance: TA)        Uplink-signal synchronization timer (Time Alignment Timer: TAT)        
The transmission timing adjustment value TA is information indicating a value for a radio terminal to advance or delay the current transmission timing by a predetermined amount. The synchronization timer TAT indicates a duration for which the timing of receiving an uplink signal at a radio base station is accommodated within a predetermined window, that is, uplink-signal synchronization is guaranteed, with a transmission timing currently configured. The radio terminal is enabled to transmit uplink signals while the synchronization timer TAT is running, but does not transmit (is disabled to transmit) uplink signals when the synchronization timer TAT expires.
Moreover, in LTE-Advanced (LTE-A), which is a radio communication system advanced from LTE, the standardization of carrier aggregation (CA) is being proceeded, in which radio terminals use multiple component carriers (CC) at the same time to transmit and receive user data and the like (NPL 2). Each component carrier CC corresponds to one system bandwidth defined in LTE and can be thought to correspond to one cell. That is, a downlink component carrier CC and a corresponding uplink component carrier CC in combination are thought to be one cell. For example, transmission and reception on two downlink (or uplink) component carriers CC can be translated to transmission and reception on two cells. Accordingly, communication using a single uplink/downlink component carrier CC corresponds to communication on a single cell, and in the description hereinafter, both or one of the component carrier and the cell will be used appropriately.
Here, a component carrier CC that performs the most basic functions such as obtaining system information required for a radio terminal to communicate with a radio base station, is referred to as primary component carrier (Primary CC: PCC) or primary cell (PCell), and other component carriers are referred to as secondary component carrier (Secondary CC: SCC) or secondary cell (SCell).
In LTE-A, studies have hitherto been proceeded on the premise that when carrier aggregation CA is performed, common uplink-signal transmission timing is used on multiple component carriers CC or multiple cells. That is, even when uplink signals are transmitted by using multiple component carriers (multiple cells corresponding thereto), there is one uplink transmission timing adjustment value TA that a radio base station notifies to a radio terminal at certain time, and there also is one synchronization timer TAT for each radio terminal. Thereby, it is possible to easily perform uplink-signal transmission timing control without complicating the control even when carrier aggregation CA is performed.
On the other hand, in 3GPP, studies of a technology improved from the technology standardized as LTE-A have been started. Specifically, the technology has been discussed that makes carrier aggregation CA feasible even if uplink-signal transmission timing differs between a plurality of component carriers CC, that is, a plurality of cells. Factors causing uplink-signal transmission timing to differ between a plurality of component carriers CC (a plurality of cells) are different frequency bands, a repeater (repeating station) being set for each frequency band (or only for a specific frequency band), and the like.
In 3GPP, a group of one or a plurality of component carriers (cells) on which uplink-signal transmission timing can be controlled in common is referred to as synchronization group (Timing Advance Group: TA Group). NPL 3 proposes a method for controlling synchronization timers, in which the uplink-signal transmission timing adjustment value TA is controlled for each such TA Group on which timing control can be performed in common, and in which one synchronization timer TAT is maintained also for each TA Group.
Hereinafter, a brief description will be given of a method for controlling synchronization timers in a case where uplink-signal transmission timing differs between a plurality of component carriers CC (a plurality of cells), with reference to FIGS. 1 to 3.
Referring to FIG. 1, a system will be considered in which two TA Groups 1 and 2 with different uplink-signal transmission timings exist. Here, it is assumed that a primary cell PCell and a secondary cell SCell1 belong to the same TA Group 1, and secondary cells SCell2 and SCell3 belong to the same TA Group 2 as shown in FIG. 2A, and that to a radio base station eNB, a radio terminal UE performs uplink transmission by using the three secondary cells SCell1-3 in addition to the primary cell PCell. In this case, since uplink transmission timing differs between the TA Groups 1 and 2, uplink transmission timing adjustment values TA1 and TA2 for the respective TA Groups are configured so that uplink signal reception timing at the radio base station eNB will be accommodated within a predetermined window as shown in FIG. 2B.
Moreover, referring to FIG. 3, according to NPL 3, a radio terminal UE controls a synchronization timer TAT1, linking it with uplink-signal transmission timing control on the TA Group 1, and similarly controls a synchronization timer TAT2, linking it with uplink-signal transmission timing control on the TA Group 2. That is, when uplink-signal synchronization is established in the individual TA Groups (TA Groups 1 and 2), the respective corresponding synchronization timers TAT1 and TAT2 are started, and each time transmission timing adjustment values TA1 and TA2 are received while the synchronization timers TAT1 and TAT2 are running for the respective TA Groups, the synchronization timers TAT1 and TAT2 are restarted (restarted from a set value) (Steps S11 and S12). According to this timer control, the radio terminal UE can determine whether or not uplink-signal synchronization in each TA Group is guaranteed.
Moreover, it is proposed that when the synchronization timer TAT1 for the TA Group 1 expires (Step S13), the synchronization timer TAT2 is stopped (Step S14) even if the synchronization timer TAT2 is running at that time, that is, even if uplink signals in the TA Group 2 are in synchronization. This proposal is based on the restriction according to CA in LTE-A that a radio terminal UE can transmit uplink control signals (Physical Uplink Control Channel: PUCCH) only on the primary cell. That is, when an uplink signal on the primary cell, on which uplink control signals (PUCCH) can be transmitted, is not in synchronization, a radio terminal UE should stop transmission of all uplink signals, and the proposal aims to easily implement this by configuring the synchronization timer TAT2 to be stopped upon expiry of the synchronization timer TAT1.
Note that after the synchronization timer TAT2 is stopped, the radio terminal UE does not restart the synchronization timer TAT2 even when a transmission timing adjustment value TA2 for the TA Group 2 is received. Thereby, it is possible to avoid a situation where the radio terminal UE transmits an uplink signal in the TA Group 2 when the synchronization timer TAT1 expires but the synchronization timer TAT2 is running.